Polymers that absorb aqueous liquids, so-called superabsorbers, are known from numerous publications. Modified natural polymers as well as partially or completely synthetic polymers may be used for this purpose. The fully synthetic polymers are, as a rule, produced by free-radical polymerisation of various hydrophilic monomers in aqueous solution according to different methods. In general, crosslinking agents are incorporated by polymerisation, whereby the polymer obtained is no longer water-soluble but is only water-swellable. For example, polymers based on (meth)acrylic acid that are present in partially neutralised form as alkali metal salt may be used as superabsorbers.
The superabsorber polymer is, as a rule, mechanically comminuted (reduced to a powder), dried and ground after the polymerisation. In this connection the pulverulent, water-swellable polymer falls in a more or less broad grain spectrum depending on the production process, which is typically in a range from 10 to 1000 μm, of which normally the grain fraction from 150 to 850 μm is used for practical purposes, above all in the hygiene sector, as absorption material. Fine fractions of less than 150 μm are undesirable on account of their dust-forming behaviour and their toxic properties when inhaled.
The development of more recent nappy constructions is highlighted by the tendency to replace increasingly larger proportions of the voluminous cellulose fluff by superabsorbers. This is happening on grounds of volume reduction and of an improved property profile. Due to the increased concentration of superabsorbers there is increased contact of the swollen absorber particles with one another after absorption of liquid has taken place. By means of the surface post-crosslinking methods described in the prior art, the so-called gel blocking, in which only the surfaces of the absorber particles swell and the liquid does not penetrate into the inner regions and swollen absorber particles that have clumped together build up to form a barrier layer for subsequent liquid, can be suppressed.
The superabsorbers are therefore post-crosslinked on the surface after the comminution, drying, grinding and grading.
Such surface post-crosslinking processes are described for example in patent specifications DE 40 20 780 C1 and U.S. Pat. No. 4,043,952. In DE 40 20 780 C1 the polymers are crosslinked on the particle surface by low molecular weight organic compounds. Not only is the absorptive capacity under pressure thereby raised, but also the behaviour of the absorbers known as “gel blocking” is suppressed. U.S. Pat. No. 4,043,952 discloses a treatment of the surface of the absorber particles with at least divalent metal ions (column 8, line 51) in organic solvents in order to improve the dispersibility in aqueous media and effect a more rapid absorption of the liquid. EP 233 067 B1 and U.S. Pat. No. 4,558,091 describe the surface post-crosslinking of superabsorbers based on polyacrylic acid and/or hydrolysed starch/acrylonitrile graft polymers with aluminium compounds in combination with polyhydroxyalcohols in order to improve the absorption properties.
During and after the surface post-crosslinking the polymer powders are changed by mixing and transportation processes as regards their grain spectrum, due to the formation of finely particulate abraded material, with the result that a renewed screening of the fine fractions is necessary in order to restore the previous state. This results in additional production costs and material losses since the fine fractions can now no longer be used at all, or at best only to a limited extent. In addition to the formation of the abraded material, there is also a deterioration of the absorption properties that had previously been improved by the surface post-crosslinking, i.e. in particular the ability of the swollen absorber gel to transport further liquid (gel permeability) is also impaired. This problem occurs not only in the production of the superabsorber powders, but ultimately also in their subsequent further processing for the production of hygiene products. In this case it is frequently found that the absorption properties of the superabsorbers are impaired due to abrasion during their conveyance, and undesirable dust is formed.
In EP 691 995 A1 describes measures for example for reducing the proportion of dust by addition of polyglycols, which however only prevent the dust but do not deal with the problem of the deteriorated absorption properties. U.S. Pat. No. 5,002,986 describes a process for improving the absorption rate of superabsorbers that consist only of fine fractions that are agglomerated in intensive mixers in the presence of ionic crosslinking agents to form larger particles. DE 196 46 484 A1 describes superabsorbers consisting of a combination of special crosslinking agents and monomers that suffer only a slight deterioration in properties under mechanical stress. A significant suppression of the property loss cannot however be achieved in this way.
In the prior art, no superabsorbing polymer is known whose properties are not impaired by mechanical stress during conveyance in production and nappy manufacture. The prior art also does not disclose any process that solves the problem of impairment of the properties of the superabsorber powders due to the mechanical stress during the surface modification and subsequent conveyance in production and nappy manufacture. In any case, a screening of fine fractions is carried out at the expense of product yield.
The object of the present invention is to provide polymers whose properties deteriorate only insignificantly during nappy manufacture, that do not form dust or only small amounts of dust, and that have a lesser tendency to form clumps in environments with a high atmospheric moisture content than products of the prior art. The object of the present invention is also to provide a process for the production of the aforementioned polymers, by means of which the screening of the fine fractions after the surface post-crosslinking is largely avoided without the properties of the polymer being substantially impaired. A further object of the present invention is to provide a substance by means of which the gel permeability of surface-crosslinked superabsorber powders containing fine fractions and that have been damaged by abrasion processes is restored.